1. Field of the Invention
The present invention relates generally to electrophotographic copying and printing and, more particularly, to unique techniques for substantially reducing edge raggedness of printed characters.
2. Description of the Invention
While the electrophotographic copying and printing industry has grown enormously in the past 30 years, and the quality of the images have significantly improved during this time frame, its images can still be readily recognized as inferior to offset printed images.
Technical innovation in electrophotography has made possible several advances in copy quality. First, by changing the development system from cascade to insulative magnetic brush development in the early 1970's, it became possible to copy and print solid blacks. Second, the blackness of the images was increased while the background was decreased by introducing the conductive magnetic brush development system. Yet close inspection of images made today with electrophotography reveal two obvious defects as compared to images made by offset printing techniques which can be obtained in any magazine, book, or advertisement. First, laser electrophotographic printers have stairstepping along diagonal lines. Second, for both electrophotographic printers and copiers, the images are "fuzzy".
The stairstepping that occurs in laser electrophotographic printers results from the finite positioning of the laser beam. It can be corrected by increasing the resolution of the laser beam, that is, providing more dots per inch, or by allowing the software to turn on the laser beam at subpixel positions. Both techniques are now available commercially.
The fuzziness apparent on all output from electrophotographic printers and copiers is a well-recognized defect. Various "solutions" based on changing the photoreceptor and the paper have been suggested. The inventors, however, have looked elsewhere for a solution.
To allow laser-printers to effectively compete with offset printing, the quality of the laser printer output must be made comparable to the quality of offset printing. The major unresolved quality difference today between these two printing technologies is the "fuzziness" of the images.
The technical term for "fuzziness" is edge raggedness. There are various ways to measure edge raggedness. The one chosen for purposes of the invention includes the steps of (1) capturing the image in computer memory using a CCD camera enabling its display on a television screen, (2) identifying the edge using an algorithm which locates where the black-white transition occurs, (3) defining a least squares straight line through the edge, and (4) calculating the mean square deviation of the actual edge from the best straight line.
Using this definition of edge raggedness, it has been found that offset images have an edge raggedness of about 5 .mu.m, while electrophotographic images from systems with magnetic brush development have an edge raggedness of 17 .mu.m and systems using monocomponent development have an edge raggedness of 12 .mu.m. These differences are easily perceived by customers as an increased "fuzziness" around characters. With a low power magnifying glass, the eye can perceive toner scattered around the character, which is clearly associated with the character. The technical problem is therefore to identify the source(s) of the toner scatter and to eliminate them. As far as is known, no one has attempted a systematic study of this problem. Only a systematic analysis of the problem will allow the identification of the contributing subsystems, their mechanisms, and appropriate solutions. The inventors have addressed this problem and have found that the sources of the edge raggedness are two subsystems in the electrophotographic system: the development subsystem and the transfer subsystem.
All of the subsystems in electrophotography could in principal also contribute to edge raggedness, and perhaps do under various circumstances. For example, surface conductivity on the photoreceptor can lead to charges in the latent image spreading out in time, perhaps leading to ill-defined edges. Or, during fusing, the unfused toner could be disturbed before it is fused into a melted image. However, evidence for these effects have not been found by the inventors. Of the six subsystems in electrophotography, namely, charging, exposure, development, transfer, fusing, and cleaning, the ones found by the inventors to contribute to the edge raggedness are the development and transfer subsystems.
It was in light of the foregoing state of the art that the present invention has been conceived and is now reduced to practice.